Processes for patterning semiconductor wafers typically rely on lithographic transfer of a desired image from a thin-film of radiation-sensitive resist material. The process entails the formation of a sacrificial layer, the “resist”, which is photo-lithographically patterned. Generally these resists are referred to as “photoresists”.
The patterning of the resist involves several steps, including exposing the resist to a selected light source through a suitable mask to record a latent image of the mask and then developing and removing selected regions of the resist. For a “positive” resist, the exposed regions are transformed to make such regions selectively removable; while for a “negative” resist, the unexposed regions are more readily removable.
The pattern can be transferred into surface texture in the wafer by etching with a reactive gas using the remaining, patterned resist as a protective masking layer. Alternatively, when a wafer is “masked” by the resist pattern, it can be processed to form active electronic devices and circuits by deposition of conductive or semiconductive materials or by implantation of dopants.
Materials used in single layer photoresists for optical lithography should meet several objectives. Low optical density at the exposure wavelength and resistance to image transfer processes, such as plasma etching, are two important objectives to be met by such a photoresist material. Shorter wavelengths of radiation permit greater resolution. The most common wavelengths currently used in semiconductor lithography are 365 nm and 248 nm. The desire for narrower linewidths and greater resolution has sparked interest in photoresist materials that can be patterned by even shorter wavelengths of light.
All manufacturing of integrated circuits has been enabled by high-performance spin-on organic polymeric photoresists. If organic resists continue in their critical role within the ever shrinking resolution demands of advanced lithography, the stringent process window demands at sub-100 nm resolution will need to be met by resists possessing sufficient sensitivity to meet the equally demanding required throughput.
Resists should maintain critical linewidth control throughout the patterning process, including both imaging and subsequent transfer via plasma etch. Line-edge roughness on the order of 5 to 10 nm is a concern at 250 nm, but will render a lithographic process unworkable when critical dimensions fall to below 100 nm. Furthermore, transferring imaged patterns via plasma etch requires that a sufficient resist be present to act as an etch mask, but single layer resists appear to be limited by an aspect ratio of 3:1. As critical features approach 25 nm, resist thickness is expected to drop to under 100 nm, a thickness that does not allow plasma image transfer even with a several fold improvement in plasma etch selectivity.
Unless plasma etch selectivity increases several fold (an unlikely event with organic based resists) single layer resist chemistry will cease to be practical at sub- 100 nm resolution. Multilayer resist schemes offer the capability of increased aspect ratio, but they add to the process complexity and cost. Therefore, a need exists to provide photoresists which meet these challenging demands.